The present invention relates to a process and apparatus for the production of synthesis gas, particularly for but not necessarily limited to, use in the production of hydrocarbon oils and waxes using the Fischer-Tropsch (xe2x80x9cF-Txe2x80x9d) process or methanol by catalytic hydrogenation of carbon monoxide.
Natural gas may be found in remote locations both on- and offshore. It is generally expensive and impractical to transport natural gas from its source to a distant processing plant. One solution is to convert the gas on-site to a valuable and easily transportable product. In this way, the value of the natural gas may be increased.
Natural gas may be converted to xe2x80x9csynthesis gasxe2x80x9d which is a mixture of carbon monoxide and hydrogen. Synthesis gas may be converted to a solid or liquid synthetic fuel or xe2x80x9csynfuelxe2x80x9d. The synfuel has less volume per unit mass (i.e. has a greater density) than the natural gas. Accordingly, it is more economical to transport synfuel than a corresponding amount of natural gas.
One disadvantage of the onsite processing of natural gas is that the space available for the processing apparatus is often limited. For example, in situations where the source of natural gas is offshore, a gas rig or a sea vessel is used to support the apparatus for extracting the natural gas. The processing apparatus required to convert natural gas into synfuel must be as compact and as lightweight as possible without sacrificing efficiency, productivity or cost-effectiveness. A further disadvantage is that the remote locations of the processing plants require that the plants are as self-sufficient as possible in the production of power to drive associated apparatus.
Examples of synfuels include high molecular weight hydrocarbon compounds produced using the F-T process and methanol produced by the catalytic hydrogenation of carbon monoxide, Between 50 and 60% of the total cost of an F-T liquid or a methanol plant is in the production of the synthesis gas. Clearly, if the cost effectiveness of the synthesis gas generation process is adversely effected in attempting to overcome these disadvantages, the overall processing costs of synfuel production could be significantly increased.
There are several methods of producing synthesis gas from natural gas. Three such methods are based on the following processes:
Steam methane reforming (xe2x80x9cSMRxe2x80x9d) which needs imported carbon dioxide or the consumption of excess hydrogen to achieve the required 2:1 ratio for the relative proportions of hydrogen and carbon monoxide in the resultant synthesis gas.
Partial oxidation (xe2x80x9cPOXxe2x80x9d) of natural gas with pure oxygen which achieves a hydrogen to carbon monoxide ratio in the resultant synthesis gas of from 1.6 to 1.8:1.
Autothermal reforming (xe2x80x9cATRxe2x80x9d) which consists of a partial oxidation burner followed by a catalyst bed with a feed of natural gas, steam and oxygen to produce the required 2:1 ratio for the relative proportions of hydrogen and carbon monoxide in the resultant synthesis gas.
Each of these three processes produces high temperature synthesis gas (SMR 800 to 900xc2x0 C., POX 1200 to 1400xc2x0 C. and ATR 900 to 1100xc2x0 C.). The excess heat generated in these processes may be used to generate steam which, in turn, can be used in steam turbines to drive air separation systems, air compressors and other equipment. The excess may also be used in part in a secondary gas heated catalytic reformer (xe2x80x9cGHRxe2x80x9d). For a POX/GHR combination, the synthesis gas is typically produced at 500-600xc2x0 C.
Carbon dioxide and methane are well known to have xe2x80x9cgreenhouse gasxe2x80x9d properties. It is, therefore, desirable that processes for the production of F-T liquids and methanol have low emission levels of these greenhouse gases and other pollutants, for example, oxides of nitrogen (xe2x80x9cNOxxe2x80x9d).
It is, therefore, desirable that the processing of natural gas to produce F-T liquids or methanol using synthesis gas is as efficient in terms of yield and capital and running costs as possible with minimal emissions and power wastage. In addition, the plant should be compact and lightweight, particularly if located offshore.
Various attempts have been made to develop processes displaying at least some of these desiderata. Attempts to integrate certain steps of the component processes are known to achieve some of these goals. Examples of such attempts are disclosed in WO-A-0003126 (Fjellhaug et al), WO-A-9832817 (Halmo et al) and WO-A-0009441 (Abbott).
U.S. Pat. No. 4,132,065 (McGann; published 2nd January 1979) discloses a continuous partial oxidation gasification process for producing synthesis gas. A hydrocarbonaceous fuel such as natural gas is reacted with a free oxygen containing gas, preferably air, optionally in the presence of a temperature moderator such as steam or water to produce synthesis gas. A portion of the synthesis gas is combusted in the presence of compressed air to produce a combustion product gas which is expanded in a gas turbine. Free oxygen containing gas is provided by a compressor that is driven by at least a portion of the power generated by the expansion of the combustion product gas in the gas turbine.
It is the primary objective of this invention to improve the efficiency and lower the capital and operation costs of a synthesis gas generation process. A further objective of the invention is to reduce greenhouse gas emissions from such a process. The process is to have particular application in the production of synfuels.
It has been found that, by integrating a synthesis gas generation process with a gas turbine producing power, at least a portion of which may be used to drive a cryogenic air separation unit (xe2x80x9cASUxe2x80x9d), process efficiency can be increased and process cost reduced. In addition, greenhouse gas emissions can be reduced and the plant can be made more compact and lightweight. Further, there is an improvement in the level of self-sufficiency in respect of power generation.
Hydrocarbon fuel gas is reacted with steam and/or oxygen gas in a synthesis gas generation system to produce a synthesis gas product stream. An oxidant gas is compressed to produce a compressed oxidant gas, at least a portion of which is combusted in the presence of combustion fuel gas to produce combustion product gas. The combustion product gas is expanded to produce power and expanded combustion product gas. Heat from the expanded combustion product gas is recovered by using the expanded combustion product gas to heat steam by heat exchange to produce heated steam, at least a portion of which is used to provide at least a portion of any steam requirement for producing the synthesis gas product stream in the synthesis gas generation system. Additionally or alternatively, at least a portion of the oxygen gas is provided using an ASU that is driven by at least a portion of the power generated by the expansion of the combustion product gas.